Disease

[PPIB_HUMAN] Defects in PPIB are the cause of osteogenesis imperfecta type 9 (OI9) [MIM:259440]. OI9 is a connective tissue disorder characterized by bone fragility, low bone mass and bowing of limbs due to multiple fractures. Short limb dwarfism and blue sclerae are observed in some but not all patients.[1][2]

Function

[PPIB_HUMAN] PPIases accelerate the folding of proteins. It catalyzes the cis-trans isomerization of proline imidic peptide bonds in oligopeptides.

Evolutionary Conservation

Publication Abstract from PubMed

Little is known about how chaperones in the endoplasmic reticulum are organized into complexes to assist in the proper folding of secreted proteins. One notable exception is the complex of ERp57 and calnexin that functions as part the calnexin cycle to direct disulfide bond formation in N-glycoproteins. Here, we report three new complexes composed of the peptidyl prolyl cis-trans isomerase cyclophilin B and any of the lectin chaperones: calnexin (CNX), calreticulin (CRT), or calmegin (CMG). The 1.7 A crystal structure of cyclophilin with the proline-rich P-domain of CMG reveals that binding is mediated by the same surface that binds ERp57. We use NMR titrations and mutagenesis to measure low micromolar binding of cyclophilin to all three lectin chaperones and identify essential interfacial residues. The immunosuppressant cyclosporin A did not affect complex formation, confirming the functional independence of the P-domain-binding and proline isomerization sites of cyclophilin. Our results reveal the P-domain functions as a unique protein-protein interaction domain and implicate a peptidyl prolyl isomerase as a new element in the calnexin cycle.